In completing oil and gas wells, it is common practice to set a string of pipe, known as casing, in the well and place cement around the outside of the casing to isolate the various hydrocarbon bearing formations penetrated by the well. To establish communication between the hydrocarbon bearing formations and the interior of the casing, the casing and its cement sheath are perforated. After the well is completed, hydrocarbons are produced from the subterranean formations.
During the course of hydrocarbon production from the subterranean formation, the rate of production may decline or be substantially reduced for a variety of reasons. For example, most recoverable hydrocarbons may have been produced from a particular zone in the formation or the reservoir pressure may have become so low that the hydrocarbons no longer flow at a sufficient rate. In other cases, like miscible floods with a gas, an early breakthrough of the gas may occur resulting in no hydrocarbon production. In such instances of reduced production, it may be desirable to shut off the perforations with cement and to reperforate lower or higher in the formation to produce the hydrocarbons. The present invention is particularly related to and useful in remedial cementing to shut off perforations. It is also useful to plug off channels behind the casings. If these channels are not plugged, injected fluids may migrate to unintended zones. In production wells, unplugged channels behind the casing may cause the production of undesired fluids from unintended zones.
Typically, when perforations are to be sealed, a remedial cementing operation is performed in which cement is simply pumped into the well casing and squeezed through the perforations until it contacts the surrounding formation. When channels are to be plugged, the cement squeeze operations are continued until a plug is formed in the channel. However, it is quite common that such cement squeeze operations to seal perforations and/or plug channels have to be repeated several times before a satisfactory seal or plug is obtained. As is readily apparent, the costs associated with well workover and lost production time increase each time a cement squeeze operation has to be repeated. Therefore, there exists in the industry a need to efficiently and effectively seal wellbore perforations and plug channels behind casings.